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Sep 26, 2023 at 17:21 vote accept Jason Li
Sep 26, 2023 at 6:09 history edited Willie Wong CC BY-SA 4.0
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Sep 26, 2023 at 6:07 comment added Willie Wong Another way to think about items 1 and 2: your manifold inherits a product Riemannian structure and hence a Levi-Civita connection, with respect to which you can define the Hessian of $f$. The musical operations allow you to consider the operation $H^\sharp_f: T(M\times N) \to T(M\times N)$, which is self-adjoint since the Levi-Civita connection is torsion free. The product structure gives the canonical decomposition $T_{x,y}(M\times N) = T_xM \times T_y N$ and your cross derivative is just the off-diagonal parts of $H^\sharp_f$.
Sep 26, 2023 at 6:01 history answered Willie Wong CC BY-SA 4.0